In internal combustion engines, the bearing assemblies typically each comprise a pair of halfbearings retaining a crankshaft that is rotatable about an axis. Each half bearing is a hollow generally semi-cylindrical bearing shell, and typically at least one is a flange half bearing, in which the bearing shell is provided with a generally semi-annular thrust washer extending outwardly (radially) at each axial end. In other bearing assemblies it is also known to use an annular or circular thrust washer.
The bearing surfaces of bearing shells and thrust washer generally have a layered construction, in which a strong backing material is coated with one or more layers having preferred tribological properties to provide a bearing surface that faces a cooperating moving part, a crankshaft journal, in use. Known bearing shells have a substrate comprising a backing coated with an overlay layer, and commonly an intervening lining layer.
The strong backing material may be steel, having a thickness of about 1 mm or more. A known lining layer may be a copper-based material (e.g. copper-tin bronze) or an aluminium-based material (e.g. aluminium or aluminium-tin alloy), which is adhered to the substrate (either directly to the backing or to an optional interlayer). The thickness of the lining layer is generally in the range from about 0.05 to 0.5 mm (e.g. 300 μm of copper-based alloy of 8% wt Sn, 1% wt Ni, and balance of Cu, apart from incidentally impurities).
The overlay layer may be 6 to 25 μm of a plastic polymer-based composite layer or a metal layer (e.g. a tin-based alloy overlay layer), which may be deposited by spraying, sputtering or electroplating.
A particular challenge to the performance of bearing lining layers and/or overlay layers is provided by the fuel-saving configuration of vehicle engines to “stop-start” operation, in which the engine is stopped and requires restarting each time the vehicle stops, in contrast to conventional engine operation, in which the engine is kept running throughout a vehicle's journey. Engines configured for stop-start operation may restart their engines about one hundred times more frequently than conventionally configured engines, which are run continuously throughout each vehicle journey. Although engine bearings are conventionally hydrodynamically lubricated during use, typically little or no lubrication is provided to the bearings when the engine starts. Accordingly, stop-start operating of an engine can place increased demands upon the performance of the bearings.